In ink-jet printing,tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores the printing data electronically and controls a mechanism for ejecting the ink drops image-wise onto the ink-receiver. Printing can be accomplished by moving a print head across the ink-receiver or vice versa.
The ink fluids can be roughly divided into:                water based, the drying mechanism involving absorbance, penetration and evaporation;        oil based, the drying involving absorbance and penetration;        solvent based, the drying mechanism involving penetration but primarily evaporation;        hot melt or phase change, in which the ink is liquid at the ejection temperature but solid at room temperature and wherein drying is replaced by solidification;        radiation curable, in which drying is replaced by polymerization.        
Water based, oil based and solvent based inks are jetted on ink-receivers, which typically contain either one or more porous layers that imbibe the ink via capillary action, or one or more polymer layers that swell to absorb the ink. Hot melt and radiation curable inks are usually jetted on substantially non-absorbing ink-receivers. Hot melt inks are limited to thermally stable ink-receivers, while radiation curable inks can be jetted on a wide variety of ink-receivers.
The main problem of radiation curable inks is that the image quality tends to change with the selection of the ink-receiver. In particular, the spreading of an ink droplet on the ink-receiver is highly dependent on the type of ink-receiver chosen.
One method to obtain a consistent image quality with a wide variety of ink-receivers would be to adapt the ink-jet ink set each time to the chosen ink-receiver. However, changing inks in printer and print head is very time consuming and not really a viable solution for an industrial printing environment. Therefore, the general approach is to modify the surface chemistry either with a suitable surface layer coating or by pre-treatment, i.e. plasma or corona treatment.
Corona discharge treatment and plasma treatment increases the cost, complexity and maintenance of the equipment used to process the substrates. Substrates may contain significant impurities or irregularities that may interfere with the treatment of the substrate. Thus, it is desirable to avoid the plasma treatment process where possible.
The other possibility for using the same ink-jet ink set on different ink-receivers is the application of a surface layer prior to jetting the radiation curable ink-jet ink. Generally, radiation curable ink-jet ink is jetted onto a dry surface layer, or alternatively, radiation curable inks are all jetted on a liquid layer (i.e. without intermediate curing of the liquid layer), as for example in U.S. Pat. No. 6,720,042 (3M).
U.S. Pat. No. 6,720,042 (3M) discloses an article comprising:                a) a sheet having a primed surface portion; and        b) a radiation cured ink-jetted image derived from an ink composition comprising at least 25 weight percent of at least one radiation curable monomer disposed on said primed surface portion;        wherein the article is durable for outdoor usage.        
In so-called “wet-on-wet printing”, a radiation curable ink droplet is deposited on a previously deposited, uncured radiation curable ink droplet or droplets which form a wet ink layer.
WO 03074619 A (DOTRIX & SERICOL) discloses a progressive dot printing ink-jet process comprising the steps of applying a first ink drop to a substrate and subsequently applying a second drop on to the first ink drop without intermediate solidification of the first ink drop, wherein the first and second ink drops have a different viscosity, surface tension or curing speed.
By printing wet-on-wet, the spreading of the second ink drop on the first ink drop can be well controlled, as it is also the case for a possible third and fourth ink drop. However, the spreading of the first ink drop on the substrate remains critical and is dependent on the surface properties of the substrate. Using colourless ink for the first ink drop can reduce this image quality problem. Suitable radiation curable inks, including a colourless ink, for wet-on-wet ink-jet printing are disclosed by U.S. Pat. No. 6,550,905 (DOTRIX).
At the exhibition DRUPA 2004 in Dusseldorf, Germany, the company Aellorra™ Digital presented an ink-jet printing process with a high viscous white wet layer, produced by jetting a UV-curable white ink, instead of a colourless wet layer. A second radiation curable ink was jetted on top of the white wet layer and the UV-curing was performed.
Another problem associated with radiation curable ink-jet printing is that images exhibit a poor gloss compared to solvent or aqueous based inks on an absorbing substrate. The amount of solids, i.e. the radiation curable compounds and colorants, deposited on an ink-receiver varies with the image information, resulting in a higher surface roughness and hence a reduced glossiness.
WO 0030856 (XAAR) discloses a method of ink-jet printing on a substrate, comprising the steps of forming a wet undercoat layer on the substrate; depositing onto the undercoat layer, whilst the undercoat layer remains wet, a pattern of wet ink droplets and subsequently transforming the undercoat layer and deposited ink droplets to a dry state.
WO 0030856 (XAAR) improves the print quality by varying the thickness of the undercoat inversely with the thickness of the ink, so that a flat print surface is achieved. Beside restrictions on the arrangements of print heads and the calculating power required to achieve the variation of thickness in accordance with the image to be printed, it is also difficult to avoid the spreading of undercoat layer from unprinted area's, i.e. full thickness of the undercoat layer, into the area's printed with 100% ink, i.e. zero thickness of undercoat layer, which results in less sharp images.
The spreading of ink droplets on a substrate largely defines the resolution that can be obtained. Although surface property modification by either coating or pre-treatment techniques has been widely employed, the exact nature of the ink-media interaction is not fully understood. Attempts are typically made to correlate the print quality to measurable surface parameters such as surface energy and surface roughness, but these parameters do not fully capture the behaviour of ink droplets on various media.
Therefore, it would be desirable to have a printing process wherein the resolution of an image can be accurately controlled on a wide variety of ink-receivers and whereby the image exhibits a high glossiness.